In the manufacture of semiconductor elements, the conventional so-called dry oxygen oxidation method of silicon oxide film coating by thermal oxidation has been largely replaced by the moisture oxidation process, which is also called the wet oxygen oxidation method and the steam oxidation method. The moisture oxidation method provides a silicon oxide film which is superior to that obtained by the dry oxygen oxidation method in such properties as insulation strength and masking effect.
Applicants had earlier developed a reactor for the generation of moisture by the aforesaid moisture oxidation method, as illustrated in FIG. 10. This was a supply source of super high-purity water for use in silicon oxide film coating. The reactor was disclosed in unexamined Japanese patent application No. 08-242246. Reactor 1 as shown in FIG. 10 comprises heat-resistant reactor structural components 2 and 3, a gas feed joint 4 and a moisture gas take-out joint 5 provided on reactor structural components 2 and 3, a reflector unit 9 on the inlet side provided inside first reactor structural component 2 opposite a feed gas passage 4a and a reflector unit 12 on the outlet side provided inside the second reactor structural component 3 opposite a moisture outlet passage 5a, a diffusion filter 10 provided between the two reactor structural components 2 and 3, and a platinum-coated catalyst layer 13 provided on an inside surface of the second reactor structural component 3.
The platinum-coated catalyst layer 13, which is formed on the inside wall surface of reactor structural component 3, is of a double layer construction, having a barrier coat 13a with a platinum coat 13b formed thereupon. The barrier coat 13a is formed of a nitride such as TiN, on which the platinum coat 13b is fixed by a vapor deposition technique or an ion coating technique.
In the arrangement depicted in FIG. 10, hydrogen and oxygen are fed into reactor 1 through gas feed passage 4a and diffused by a gas diffusion means 8 which comprises the inlet reflector unit 9, the filter 10, and the outlet reflector unit 12. The hydrogen and oxygen then come into contact with platinum-coated catalyst layer 13. Upon coming into contact with the platinum-coated catalyst layer 13, hydrogen and oxygen are enhanced in reactivity by catalytic action, being transformed into what is referred to as a radicalized state. Radicalized, hydrogen and oxygen react instantaneously with one another to produce water without undergoing combustion at a high temperature.
The reactor as shown in FIG. 10 is recognized as constituting a significant advance in semiconductor manufacturing technology. The reactor can be built in a substantially reduced size but can produce super high-purity water, or a mixture gas of super high-purity water and oxygen, at the rate of 1000 sccm or cc/minute in terms of the standard conditions with a high level of reactivity and response characteristics or responsiveness. To illustrate, three cases of moisture generation will be described using a reactor 1 having the construction shown in FIG. 10. The reactor 1 used was about 134 mm in outside diameter, about 70 mm in thickness, and about 490 cubic centimeters (cc) in inside volume.
The material gases of hydrogen and oxygen are fed at three different rates: Case A, hydrogen at 1000 sccm and oxygen at 1000 sccm; Case B, hydrogen at 1000 sccm and oxygen at 500 sccm; and Case C, hydrogen at 1500 sccm and oxygen at 500 sccm. The reactor turns out approximately 1000 sccm of a mixed gas of water and oxygen in Case A, approximately 1000 scam of water in Case B, and approximately 1000 sccm of a mixed gas of water and hydrogen in Case C, all at a reactor temperature of about 400.degree. C. with a moisture generation reaction rate of approximately 99 percent.
FIG. 11 illustrates changes with time in moisture generation reaction rate at a reactor temperature of about 400.degree. C. in the aforesaid reactor. FIG. 11 demonstrates that the reactor can produce super high-purity moisture with a moisture generation reaction rate of approximately 98.5 to 99.0 percent, whether the material gas mixture is oxygen-rich as in Case A or hydrogen-rich as in Case C.
However, it has been found that reactor 1 as illustrated in FIG. 10 can be improved, particularly with regard to safety. That is, it is difficult to raise the moisture generation reaction rate to more than approximately 99.0 percent when the reactor is operated at a temperature not higher than about 400.degree. C. and at a moisture generation rate of more than approximately 1000 sccm. This leaves approximately one percent of the oxygen and hydrogen unreacted in the moisture which is generated. As a result, it is impossible to extract hydrogen-free moisture or a mixture of hydrogen-free moisture and oxygen and to eliminate the possible danger of explosive combustion of hydrogen in the oxidation chamber where the mixture is introduced.
Another problem with the configuration of reactor 1 as illustrated in FIG. 10 is responsiveness in the reaction of oxygen and hydrogen to produce moisture. Even where the moisture generation reaction rate can be raised to approximately 99.8 percent in normal operation to minimize the amount of unreacted hydrogen flowing into the oxidation chamber, the reaction rate cannot necessarily be maintained at that high level at the times when the reactor 1 is started up or shut down. There is thus still a concern that unreacted hydrogen can flow into the oxidation chamber at these times.
The region indicated by the letter J in FIG. 12 shows the moisture generation responsiveness (i.e., transitional changes in the contents of oxygen and hydrogen in the moisture generated and the amount of the generated moisture) in which feeding of hydrogen and oxygen into the reactor as shown in FIG. 10 is simultaneously (time difference=0 seconds) started and stopped. The measurements were taken using a quadrupole mass spectrometer (Q-mass spectrometer). In the region indicated by the letter J, the amount of unreacted hydrogen in the generated moisture peaked at A1 and A2, that is, at the time the feeding of the material gas mixture was started and also suspended. The measurements shown in FIG. 12 were taken with a material gas mixture of 1000 sccm of hydrogen and 600 sccm of oxygen (a 20 percent oxygen rich mixture). The reactor temperature was 400.degree. C. The nitrogen gas indicated in the Figure, fed at the rate of approximately 1000 sccm, was to purge the reactor 1. The feeding of the nitrogen gas was stopped at the time that oxygen and hydrogen feeding began, and was resumed at the same time that the supply of oxygen and hydrogen was suspended.
While it is not clearly known why the amount of unreacted hydrogen in the generated moisture reaches a peak at A1 and A2 when the feeding of hydrogen and oxygen is started and suspended, it is found that as the richness of hydrogen in the material gas mixture gets higher, the peaks A1 and A2 get higher.
That the amount of unreacted hydrogen rises when the reactor for the generation of moisture is started or stopped is a problem that cannot be ignored, because this phenomenon exposes equipment using the moisture generated, such as the silicon oxidation chamber, to danger. Accordingly, the inventors conducted extensive experiments seeking to determine why the amount of unreacted hydrogen in the moisture that was generated peaked at the time of start-up or suspension of the operation of the moisture generation reactor, and how that could be prevented.
As a result, the inventors have discovered that the hydrogen peak in the generated moisture could be minimized by starting to feed hydrogen after starting to feed oxygen at the start-up time and terminating the feeding of hydrogen before terminating the supply of oxygen at the time of suspension of moisture generation. But the problem is that if the feeding of hydrogen is delayed at moisture generation start-up time, unreacted oxygen can flow into the oxidation chamber. That will put the silicon element of an oxidation film coating process into dry oxygen oxidation. As a result, oxidation film coating by the moisture oxidation process is hindered. On the other hand, suspending the supply of hydrogen earlier than suspension the supply of oxygen has little adverse effect on the film coating process because the formation of silicon oxide film has been completed by that time, However, the loss of oxygen increases and the operating rate of the silicon oxidation chamber decreases.
The present invention addresses these problems encountered with the reactor 1 illustrated in FIG. 10.